Saw that one but there were too many differences. I used a step by step photo guide here: 2010 Mustang GT Front Door Upper Speaker Replacement
And Jazzcat, you are right. There are different scenarios to try and differing opinions but given the 'fun' of installing the rears, it's something that one will only want to do once!
I hear you on the stereo imaging but I use a multispeaker set up at home for music but it's in stereo as well. So all three rights are producing the right image and left, etc. with equal bias. In thoery, I have one stereo set in front, one in the middle and one stereo set in the rear with my butt in the middle of all of it.
So maybe one should sit in front of two and then four speakers to decide if they want to commit to the rear? Otherwise, I suggest just swap out the rear as it looks worse than it really is.